Aeromonas species are ubiquitous in the microbial biosphere. They can be isolated from virtually every environmental niche where bacterial ecosystems exist. These include aquatic habitats, fish, foods, domesticated pets, invertebrate species, birds, ticks and insects. The vast panorama of environmental sources from which Aeromonas can be encountered lends itself readily to constant exposure and interactions between the genus Aeromonas and humans.
Originally Aeromonas species were recognized to cause systemic diseases in frogs, toads, salamanders, fish, lizards, rabbits, guinea pigs, dogs, cats, chickens and pigeons. Recently, the bacteria has been shown to be a significant cause of infections associated with natural disasters (hurricanes, tsunamis, and earthquakes) and has been linked to emerging or new illnesses, including near-drowning events, prostatitis, and hemolytic-uremic syndrome 3.
Aeromonas hydrophila is a Gram-negative, non-sporeforming, rod-shaped, facultative anaerobic bacilli belonging to the family Aeromonadaceae. Ubiquitous in the environment, A. hydrophila is frequently isolated from food, drinking water, and aquatic environments. It is resistant to standard chlorine treatments, probably surviving inside biofilms. Among various Aeromonas species, A. hydrophila is most commonly involved in causing human infections such as septicemia, gastroenteritis, and necrotizing fasciitis. Isolation of this organism from water and food sources, as well as the increasing resistance of this organism to antibiotics and chlorination in water, presents a significant threat to public health 8. This bacterium is not normal inhabitants of the gut and can play a significant role in intestinal disorders in children under five years old, the elderly, and immunosuppressed people. Furthermore, the cases of septicemia are often fatal 2.
Aeromonas hydrophila can produce several disease-causing proteins, including hemolysins, proteases, adhesins, lipases/phospholipases and toxins, amongst which protein toxin aerolysin is the best characterized. Act (aerolisyn-related cytotoxic enterotoxin) and aerolysin are pore-forming proteins that are responsible for the hemolytic, cytotoxic and enterotoxic activities of A. hydrophila, being its main virulence factors. Aerolisyn pore-forming toxin is found in 75% or more of organism's strains, as well as in many other species, including A. veronii (A. sobria), A. caviae, and A. trota. In terms of mode of action, it is very similar to the staphylococcal channel forming α-toxin.
Aerolysin inserts itself into the cell membrane and forms pores (channels), through which small molecules and ions flow to trigger pathogenic changes. Some cells with receptors for this toxin are very sensitive to its action. T-lymphocytes can be killed by toxin concentrations as low as 10-11. At realtively high aerolysin concentrations red blood cells and other mammalian cells are rapidly killed due to sudden increase in intracellular Ca2+, or a decline in intracellular K2+, resulting in cell swelling and death. Some experts suggested that the toxin may induce the production of proinflammatory signalling molecules interleukin-1 beta (IL-1Β), IL-6, and TNF-α (tumor necrosis factor) in macrophages. TNFα and IL-1Β stimulate the production of the inducible nitric oxide synthase (iNOS) enzyme that, through nitric oxide (NO) production, is an essential element of antimicrobial immunity and host-induced tissue damage and cell death 3,4.
More recently, two other toxins were detected in A. hydrophila, Alt (cytotonic heat-labile) and Ast (cytotonic heat-stable) enterotoxins, but their specific roles are not clearly understood.
Aeromonas are ubiquitous in water, including chlorinated drinking water and groundwater, and in a single well, the same strain can persist for years. A. hydrophila, among two other species, constitutes 85% of all gastrointestinal and extraintestinal infections. Because gastrointestinal infections with Aeromonas species are generally considered waterborne, Aeromonas hydrophila has been placed on the United States Environmental Protection Agency Contaminant Candidate List of emerging pathogens in drinking water 9.
Two distinct types of gastroenteritis have been associated with A. hydrophila: a cholera-like illness with a watery (rice and water) diarrhea and a dysenteric illness characterized by loose stools containing blood and mucus. The infectious dose of this organism is unknown, but SCUBA divers who have ingested small amounts of water have become ill, and A. hydrophila has been isolated from their stools. The organism has frequently been found in fish and shellfish. It has also been found in market samples of red meats (beef, pork, lamb) and poultry 10. In addition to these consumable products, bivalves such as oysters and mussels are naturally bathed in estuary waters containing A. hydrophila, and through their filter-feeding process, they actually concentrate these bacteria within their meats. In addition to these major pathways, Aeromonas can also be acquired by other, less prominent routes. Recreational activities such as boating, fishing, and diving can lead to infection through major or unapparent traumas 3.